In synchronous circuit applications, the clock signal is of the utmost importance. In particular, telecommunication switching systems require dependable timing signals to operate properly and to transmit digital data signals error free. To avoid failures caused by errors such as loss of clock and loss of frame, and to facilitate system fault diagnosis and testing, redundant timing signals may be provided. By using redundant timing signals, the system may operate with a backup timing signal upon detection of erroneous conditions in the active timing signal. Craft persons may also manually swap the timing signals in order to perform system diagnostics, maintenance and/or repairs. In telecommunication systems where high-speed data are transmitted, even single bit errors cannot be tolerated. It may be seen that in order to switch from one active clock signal to the other, the clock signals must be fully synchronous in frequency and phase to avoid producing bit errors in the data transmission.
Accordingly, a need has arisen for a circuit to closely phase align and switch between two or more timing signals. Additionally, such a circuit must also compensate for the effects of varying temperature on circuit operations.